This invention relates to a method and apparatus for processing slabs which have been manufactured by continuous casting (hereunder referred to merely as "continuously cast slab)".
In particular, this invention relates to a method and apparatus of preventing the formation for cracks during hot working in the manufacture of a slab by a continuous casting process and to a method and apparatus for preventing the formation of cracks during so-called "direct rolling" or "hot charge rolling".
Steels to which this invention can be successfully applied are medium or low carbon steels containing either Si or Mn, and low alloy steels which contain at least one alloying element, such as Al, Nb, Ti, Ta, V, and B, each in an amount of less than 1%.
"Direct rolling" means a rolling process in which hot slabs manufactured through continuous casting are subjected to hot rolling immediately after continuous casting without preheating. "Hot charge rolling" means a rolling process in which hot slabs manufactured through continuous casting are rolled immediately after reheatign them slightly without cooling to room temperature.
In the manufacture of these medium or low-carbon steels and low-alloy steels using a bending-type continuous casting machine, surface cracks are frequently formed on cast slabs due to thermal stresses and bending stresses which are caused by cooling and straightening. The incidence of such cracks is especially sever with Nb-containing steels.
It is necessary to remove these cracks before proceeding to the next stage of manufacture. Usually, this requires cooling to room temperature.
Direct rolling and hot charge rolling are advantageous because they do not require cooling to room temperature nor heating to a rolling temperature from room temperature. Therefore, the formation of such cracks makes these processes impossible.
Even if cracks are not formed during casting, they are sometimes formed during rolling, i.e, direct rolling, hot charge rolling, etc. In this case, too, the formation of these cracks makes these processes impossible.
It is said that a high sulfur steel inevitably suffers from cracking during hot rolling.
Therefore, in order to carry out hot working in a continuous and inexpensive manner through direct rolling or hot charge rolling, it is desirable that the formation of cracks on cast slabs during continuous casting or during direct rolling or hot charge rolling be completely prevented. In addition, even when a continuously cast slab is cooled to room temperature and then is reheated to a hot rolling temperature, cast slabs which are free from surface cracks are advantageous since conditioning by scarfing is not necessary. Thus, in this case, too, it is desirable to completely prevent the formation of cracks in a continuous casting process.
Japanese Patent Application Laid-Open Specification No. 128255/1983 discloses a method of blowing metal shot onto a slab surface to prevent the formation of surface cracks of a continuously cast slab. However, the purposes of this method are to pressure weld the cracks, to remove extraneous matter from the surface ofa slab, and to suppress oxidation of the slab surface. Such treatment is carried out just when the slab leaves a mold and before going into guid rollers. Cracks frequently develop in the steps following the casting, e.g., during rolling. Thus, this method is not a complete solution of the problem.
Japanese Patent Application Laid-Open Specification No. 155123/1979 discloses a method of applying plastic strain to a cast slab while controlling the amount of plastic strain, the cast slab temperature, and the austenitic particle size. However, according to the experience of the inventors of this invention, it is impossible to completely prevent the formation of cracks by regulating only these factors.
Furthermore, means for imparting plastic strain, which are suggested therein, are rolling, shot-blasting, laser pulse application, and the like. These means are not sufficient to impart a satisfactory plastic strain. Namely, when rolling is applied with usual rolls to a portion of a slab which is only partially solidified, the shell of the solidified metal only becomes concave without the desired strains being formed in the skin surface of a cast slab. On the other hand, shot-blasting produces plastic strains only to a shallow depth, resulting in no remarkable effects.
Furthermore, with shot-blasting, it is troublesome to collect the shot after blasting, and therefore this process is not considered practical.
A method utilizing a laser pulse applies heat to a depth of a few dozen .mu.m so as to produce strain due to thermal differences between the surface of slab and the inner portion thereof. This method, however, is not effective with hot slabs, since it is not possible to achieve any significant thermal differences when a laser pulse is applied to a hot cast slab. The presence of coolant water on the surface of a slab also makes this process impractical.
Japanese Patent Application Laid-Open Specification No. 52442/1983 proposes a method of controlling the cooling rate in a continuous casting process so as to prevent the formation of cracks. However, according to the method disclosed therein, the cooling rate is controlled so as to be small, and it takes an extremely long time before the cooling is completed. Therefore, this method, too, is impractical.